Gas turbine load sharing system



Dec. l, 1964 H L MCCOMBS, 1R 3,159,000

GAS TURBINE LOAD SHARING SYSTEM Filed April so, 196s Zei 2.2': gl] 26A( I I /4a 74a a /20 M( wa /f United States Patent O 3,159,060 GAS TUREHNE LOAD SHARING SYSTEM Howard L. McCoxnbs, Ir., South Bend, Ind., assignor to .The Bendix Corporation, South Bend, End., a corporation of Delaware Filed Apr. 36, 1963, Ser. No. 276,775 7 fClaims. (Cl. titi-39.15)

The present invention relates to a load sharing system :for a plurality of gas turbine engines particularly of the shaft output type hereinafter referred to as turbo-shaft engines. More particularly the invention applies system responsive corrective adjustments to individual engine speed governors so that all engines within `a bank of engines driving a given load will supply substantially equal shares of the power requirements.

In order to assure maximum power capability, uniformity of wear and/or deterioration, and balanced loading it is desirable in many multiple power plant installations that each engine bear a substantially equal portion of the load.

When the engine outputs are not interconnected in a physical sense, the respective engine speed governors are able by small variations in speed between respective engines to make large compensating adjustments in engine load and thus maintain a close approach to load uniformity. However, for engines of the turbo-shaft type wherein the output shafts drive a commonV load and thus are physically connected one to another so as to rotate at equal speeds or yspeeds having a fixed ratio one to another, it is no longer possible to correct for load inequities by relative speed variations between engines. Under this condition, certain engines within a bank driving a common load will assume greatly disproportionate shares of the overall load and relative corrections via speed adjustments is not possible since all speeds will vary in unison.

Accordingly it is an object of lthe present invention to provide a lload sharing control System whereby adjustments to selected engines'in a bank of enginesdriving a common load may be made whereby all engines will bear a substantially equal share of the load.4

It is another object. of the present invention to provide control means for comparing the load produced by each given engine in a bank against the load supplied by the maximum producing engine and to` correct load distribuj tion control means in accordance with the teachings of the present invention; and j FIGURE 2 is a graph illustrating governor slope chari acteristics of the individual engines of FIGURE ,l be- `fore and after adjustment by my load sharing control system. v o

lReferring in particular to FIGURE l, there is shown a multiple engine installation having a plurality of turboshaft gas turbine engines 10a, 10b, libc and 10d. Four engines have been illustrated, however, itshould beunderstood that the present invention may be applied to any installation of two or more engines.

j The engines shown are of the turbo-shaft type (Le. gas turbine engine with shaft output). Engine lea,

which is thev same as the remaining engines in the bank,

l iirst turbine 16a.

A adsense Patented Dec. l, 1964 ICC includes a gas producing section having a compressor 12a, a combustion chamber 14a, and a compressor driving Air is received from inlet manifold l i3, compressed in compressor 12a, burned with a fuel l is operative with an axially movable contoured metering,

mixture supplied by nozzle 20a in combustion chamber 14a to produce high energy content motive uid which passes through turbine 16a which extracts a portion of the energy content sufficient to drive the compressor. The engine further includes a power output section which receives the high energy content motive Huid from the gas producing section and converts this energy to a rotational shaft output. The power outputsection consists of an output turbine 22a drivably connected to output shaft 24u. Turbine 22a extracts substantially the balance of usable energy in the motive iiuid and exhausts the spent gases to exhaust manifold 26. Similar parts of engines 10b. ltic and 10d bear the same numerical designation as those described for engine 10a except that the letter subscript is changed.

Although not necessarily limited thereto, the present invention has its greatest utility applied to multiple power lant installation where the engines are driving a common load. This is illustrated schematically by a series of load devices ZSaLZd having interconnection represented by the line '30 so that each speed of respective shafts 24a 24d is equal to one another or bears a fixed ratio thereto. The load devices may for example, bea series of generators supplying power to a common line wherein the generators are physically interconnected by common gearing or are electrically interconnectedV by means well known in the art. Additionally, it is common practice to apply two engines to a common gear box in helicopter aircraft for added `reliability wherein engine output shafts are physically connected and must rotate at the same speed.

Each engine has a fuel control system 32a-32d respectively. The fuel supplying systems may be identical and function in the manner to be described with respect to system 32a.

Fuel from a supply tank or boost pump, not shown, is transmitted by conduit 34 to high pressure pump 36a which is preferably enginedriven. High pressure fuel is delivered by pump 36a to conduit 38a whichconnects said pump to the fuel delivery nozzle 20a. A fuel metering restriction 4am -is disposed in conduit 38m and valve 42a rto define a variable area metering orice. A by-pass valve 44a is operative to maintain a constant pressure differential across metering restriction 40a by bypassing fuel through conduit 46a from the outlet side of the pump -back to its inlet side. By-pass valve 44a is controlled by 'spring loaded diaphragm 48a contained in chamber 50a. Pressure from upstream of metering restriction 40a passes through a port 52a in the sidewall of valve 44a tol act in one direction on diaphragm 48a whereas pressure downstream of metering restriction 46a is transmitted -by conduit 54a- -to the opposed side of diaphragm 48a sov that the metering head or pressure differential is applied as a controlling input to valve 44a, A predetermined pressure differential is required to balance the preload of diaphragm spring 56a. Shouldthe pressure differential tend to increase above this predetermined value it will overcome the spring, by-passing more fuel to pump inlet and restoring'the pressure differential to its 'predetermined value. Should the pressure differential decrease, valve 44a will close restricting the quantity of by-pas'sed fuel and raise the pressure differential.

By maintaining pressure differential constant, the rate yof fuel delivery through conduit 38a to nozzle 20a bemetering valve 52a and thus provides a fuel system of relative simplicity where only one variable controls fuel delivery rate. Valve 42a is controlled in position by a positioning signal supplied by fuel system 32a which in the preferred embodiment is a pneumatic control of the type described in commonly assigned US. Patent 3,040,529, I ames E. Hurtle, inventor, and issued lune 26, 1962 modified to include components to effect load sharing in accordance with the teachings herein. The control consists of a housing 69a containing a small evacuated bellows 62a secured on one end to the wall in the lower end of housing 69a and a large open bellows 64a peripherally secured to the housing side walls to divide said housing into a first chamber 66a and a second chamber 68a, the latter chamber containing bellows 62a. Bellows 62a and 64a have their movable ends interconnected by a rod 70a which is thereby moved axially in response to the sum of difference of pressure forces acting on the bellows. Rod 70a is connected via linkage '72a to meter- -ing valvev 42a to transmit this motion thereto and effect variation of fuel delivery.

Compressor' discharge pressure (or combustion chamber inlet pressure) is supplied by conduit '74a from engine lila is retarded thus relaxing spring ltlZa. The engine will momentarily be at an overspeed condition with exhaust valve 88a in its Wide open position. This causes a relatively xed pressure differential across bellows @da which is subtractive from the force on bellows 62a which in net effect is equivalent to reducing the size of bellows 62a. Thus deceleration proceeds in substantially the reverse manner of an acceleration, except of course, at much lower fuel values due to the decreased effectiveness of bellows 62a.

in order to provide means for adjusting the governor independent of throttle lilZa, I have provided a control valve Title-a operative to control the effective area `of oriiice 82a in housing Sila. The valve lltlcz is controlled axially by diaphragm 166e peripherally secured to housing Sila. Diaphragm libert is urged against stop ltlda by spring member lilla in which position valve lltlfta denes a minimum effective area opening with restriction 52a.

` Stop llZa limits the maximum effective area size.

to chamber 68a where it acts over the exterior surface of bellows 62a and the interior surface of bellows 64a. This pneumatic engine pressure is also supplied to chamber 66a via conduit 7 6a, conduit 78a, housing Sila having an orifice or restriction 82a, and conduit 84a. which connects with the source pressure 74a. Conduit 76a extends beyond its connection with conduit '78a and terminates with a pair of governor controlled exhaust valves 86a and d8a which when open exhaust to the atmosphere. Exhaust valve 86a is controlled by an overspeed governor 90a driven via connection 92a at a speed proportional to the gas producer section or turbine ida and compressor 12a. Governor 99a operates against the force of spring 94a to open exhaust valve 85a when the gas producer speed exceeds a predetermined maximum safe value. Exhaust valve 83a is controlled by an all-speed or adjustable governor 96a driven by power turbine v22a via connection 93a andk which opposes spring ltlda which is adjustable by means of throttle lever @2a. Governor 96a is operative to open exhaust valve 88a when the speed of turbine 22a approaches a value determined by the force of spring ttla. This speed value is adjustable to any desired value by lever 192m The control system thus far described is in accordance with that of Patent 3,0405529 which contains a full description of the operation thereof. summarizing this operation, during engine acceleration turbine 16a is below its limiting speed and turbine 22a below its requested speed so that both exhaust valves 86a and 38a are closed. Under this condition there is no air dow through restriction 82a and consequently no pressure drop thereacross so that pressure in chamber 66a equals that in chamber 63a neutralizing bellows 64a. Compressor discharge pressure acting on bellowsl 62a is the effective force to position metering valve 42a in a gradually opening direction as n engine acceleration proceeds and compressor pressure increases. As turbine 22a approaches its selected speed, governor 96a will urge exhaust valve 83a in an open direction causing flow through restriction 82a and a resulting pressure drop thereacross thus lowering the pressure in chamber 66a below that in chamber 68a. Decreasing pressure in chamber 66a causes large bellowsdia to become an effective controlling member opposing bellows 62a. Since bellows 64a is relatively large compared to bellows 62a and exhaust valve 83a is quite sensitive causing a large pressure dropacross restriction 82a, bellows 64a becomes thev predominate control overriding bellows 62a and causing fuel valve 42a to move upwardlyl in a fuel decreasing direction. It will be noted that an increase 0f speed of turbine 22a induces a decrease in fuel to provide the inverse relationship of fuel vs. speed required for dynamic governing. During deceleration, kthrottle 162e Variation in the effective area of orice da by valve ltifia varies the gain or sensitivity of the governing characteristic. During governing operation air iiow w-ill be in series through valves i945: and 55a such that adjustment lto the former affects the latter. Specifically,fthe smaller the effective area through restrictionza, the smaller will be the increment of position change required by valve Stia to effect a given pressure change in conduit 76a and chamber 66a. As the eifective area of orice 82a is increased, valve 33a must handle greater quantities of air and thus must move further to effect a given pressure change causing a less sensitive `or low gain governor characteristic.

The proportionality or gain of the governor is referred to in technical literature in the art as the governor droop or slope which indicates the characteristic of a graph plot of speed vs. fuel flow (or power) during governing operation. Suc-h a plot is generally a straight line'curve inclined slightly from the vertical as illustrated by the curves A, B, C, D, of FGURE 2 which are governor curves for the engines lila, lilly, lltlc and lltld respectively. rThe f closer the approach of the governor curve to the vertical j its angular deviation from a horizontal axis whereas droop would be the converse of slope or proportional to the complementary angle of the slope angle.y

Control of the position of gain or slope adjusting valve Vltlla is by means of diaphragm 1h60, which is exposed to compressor discharge pressure from engine lila over its lower surface which provides a pressure force which in cooperation with a light force from spring Mila urges diaphragm lilou against minimum stop ltla. The upper face of diaphragm ltlda is exposed to the maximum compressor discharge pressure in the bank of engines supplied through a selecting network to conduit 1113. Each engine has a compressor discharge pressure conduit 74a, '7411, 749C and '7l-id, respectively, for supplying this quantity to its respective fuel system 32a, 32h, 32C and 32d. A selecting valve lle is connected by conduits M6 and lllS to conduits '74a and Mb and contains a valve plate 120 loosely contained between closely spaced ends of conduits .lid and ylll. When the pressure in conduit lille exceeds that in conduit '118, valve plate l2@ movesto the right closing conduit 118 venting compressor discharge pressure of engine lila to conduit 122'. Should the pressure of engine ltlb be greater, valve pl-ate 12d will move to the left closing conduit llo venting compressor discharge pressure of engine liib to conduit 122. Thus valve 1M selects the greater of the compressor discharge pressures of engines lila and Mb and supplies this pressure to conduit 12.2. A similar selector valve 124 isnconnectcd between engines we and lltld to supply the greater pressure to conduit i226. A third selecting valve T128 selects the greater pressure in conduits E22 and 126 andsupplies the highest pressure in the banlr of engines to conduit 3112 where itis distributedk For consideration of the operation of my load sharing Y system, reference is made to the graph of FIGURE 2 showing four randomly oriented governor slope curves A, B, C and D for each of the respective engines. The variations between governor curves, although somewhat exaggerated for purposes of illustration, results from differences in manufacturing tolerances between engines and controls and exists in even the most closely cont-rolled conditions. Assuming all engines are driving a common load at equal speeds N1, the intersection of a vertical line drawn from N1 with the governor curves indicates the fuel delivery and approximate load each engine wo-uld deliver without load sharing. As shown there is greatly disproportionate load distribution with engines Mib and lid aS- suming a greater than average share whereas engines a and ltlc are under producing.

However, corrective adjustments are applied to the respective engine governing systems by my loady sharing control which has the effect of equally distributing the load as described hereinafter.

In the bank of engines described above, compressor discharge pressure will vary proportionately with engine load such that engine 10b will have the greatest pressure; engine ltld will be next; followed by engine 10a and finally engine 10c in proportion to the load distribution indicated in FIGURE 2. Pressure on the lower face of diaphragm 166e will be the relatively low pressure lof engine lila. The selecting network will supply pressure from engine 10b which is the maximum producing or master engine to conduit 112 where it is supplied to the respective fuel systems. Diaphragm 166e, for example, will thus compare the pressure derived from engine lita against the highest pressure in the bank of engines, or in the illustrated case against the pressure of engineV ltlb. Diaphragm wea is urged downwardly against the force of spring Illia increasing the eifective area of restriction 82a decreasing the gain or increasing the Vdroop of the governor characteristicl as illustrated in FIGURE 2 by curve A. Each engine governor characteristic will be` correspondingly adjusted in like manner except for engine lub which has equal pressure -across its control diaphragm. Engine' speed will correct to a new value N2 where all governor curves intersect at the point X f and each will produce a substantially average portion of the overall load. If after correction it is desired to run at speed N1, the throttle lever ltlZa of the engine governors may be slightly retarded, to restore the original speed.

ln the present invention compressor discharge pressure has been selected as a preferred engine parameter for indirectly Vmeasuring relative engine power. Particularly when combined with a pneumatic fuel control of the type described herein it will be observed that no separate power sensing equipment is required, but rather this measurement may simply be tapped from an existing control sensing conduit 74a and thus for the application described is the most economic means for sensing relative power. Other engine parameters lend themselves to producing relative power comparisons with varying4 degrees of accuracy, response and complexity. For installationshaving engine output shafts driving at the same speed, a torque measurement on the output shaftmaybe utilized `as a power'comparison. The temperature drop across turbine 22a varies proportionately withV power as does the speed of turbine lea although with less accuracy. It is expected that those practicing the present invention may select the most suitable parameter `*based on cost and accuracy considerations as a power 1 index without departing from the present invention de- Vfined in the appended claims. I'claim: p

1. A loadsharing system for la bank of two or more 6 turbo-shaft engines driving a'common load comprising: a governor device for each engine; each of said governor devices being responsive to the shaft speed of its associated engine and operative to control fuel delivery in response thereto; adjusting means for each engine; each of said adjusting means connected to its associated governor device and operative to adjust the governor controlled relationship between shaft speed land fuel flow; power sensing means for each engine; each of said power sensing means operative to produce an individual power signal representative of the power developed by its associated engine; selecting means connected to each of said power sensing means for selecting the maximum individual'power signal; comparison means for each engine; each of said comparison means being connected to said power sensing means of its associated engine and to said selecting lmeans to compare its associated engine individual power signal with said maximum individual power signal; each of said comparison means being connected to said adjusting means of its associated engine to adjust said governor devicek in response to deviation between said individual power signal and said maximum individual power signal.

2. A load sharing systenrfor a` bank of two or more turbo-shaft engines, each engine having a compressorV and an output shaft driving a common load comprising: a governor device for each engine; each of said governor devices being responsive to output shaft speed of its associated engine and operative to control fuel delivery in response thereto; adjusting means for each engine; each of said adjusting means connected to its associated gover- `nor device and operative to adjust the governor controlled relationshipV between shaft speed and fuel flow; power sensing means for each engine; each of said power sensing means operative to sense compressor discharge pressure of its associated engine; selecting means connected to each of said power sensing means for selecting the maximum compressor dischargev pressure sensed from said bank of engines; comparison means for each engine; each of said comparison means being connected to said power sensing V'means of its associated engine and to said selecting means to compare compressor discharge Y turbo-shaft engines, each engine having a compressor section and an output shaft driving a common load comprising: fuel control means for each engine for controlling the rate of fuel delivery thereto; each of said fuel v control means including a governor device responsive to output shaft speed of its associated engine for controlling fuel flow inversely proportional to speed of said output shaft; 'adjusting means for each engine; each of said adjusting means connected to its associated governor device and operative to adjust the proportionality constant or gain thereof; power sensing means for each engine; each of said power sensing means operative to sense compressor discharge pressure of its associated engine; selecting means connected to each of said power sensing means for selecting the maximum compressor discharge pressure sensed from said bank of engines; comparison means for each engine; each of said comparison means being connected to said power sensing means of its associated enginev and to said selecting means to compare compressor discharge pressure of its associated engine with the maximum compressor discharge pressure from said bank'of engines; each of said comparison means being connected to said adjusting means of its associated engine to adjust the gain of said governor device in response to said pressure comparison. i I

4. A load sharing system for a bank of two or more turbo-shaft engines, each engine having a compressor section and an output shaft driving a common load comprising: fuel control means for each engine for controlling the rate of fuel delivery thereto; each of said fuel control means including a governor device responsive to output shaft speed of its associated engine for controlling fuel flow inversely proportional to speed of said output shaft; adjusting means for each engine; each of said adjusting means connected to its associated governor device and operative to adjust the proportionality constant or gain thereof; power sensing means for each engine; each of said power sensing means operative to produce an individual power signal representative of the power developed by its associated engine; selecting means connected to each of said power sensing means for selecting the maximum individual power signal; comparison means for each engine; each of said comparison means being connected to said power sensing means of its associated engine and to said selecting means to compare its associated engine individual power signal with said maximum individual power signal; each of said comparison means being connected to said adjusting means of its associated engine to adjust said governor device in response to deviation between said individual power signal and said maximum individual power signal.

5. A load sharing system for a bank of two or more turbo-shaft engines, each engine having -a compressor section and an -output shaft driving a common load comprising: fuel control means for each engine; each of said fuel control means having air conduit means connected to its associated engine compressor section for a high pressure control luid source; first and second variable position air control valves arranged in series in each'of said conduit means for controlling the air pressure therethrough with said second valve downstream of said rst valve; a governor device for each engine responsive to output shaft speed; each of said governor devices; being connected to said second valve of its associated fuel control means for varying the position thereof in response to output shaft speed; adjusting means for each engine responsive to power comparison between its asso-ciated engine and the maximum producing engine within said bank of engines; 'said adjusting means being connected to said `iirst valve of its associated fuel control means for controlling the position thereof in response topower comparison; each of said fuel control means including a 'pressure responsive member connected to its associated air conduit means intermediate said first and second valves; said pressure responsive member operative to vary fuel delivery to its associated engine in response to pressure sensed in said conduit means.

6. A load sharing system for a bank of two or more turbo-shaft engines, each engine having a compressor section and an output shaft driving a common load comprising: fuel control means for each engine; each of said fuel control means having air conduitmeans connected to its associated engine compressor section for a high pressure control fluid source; iirst and second variable position air control valves arranged in series in each of said air conduit means for controlling the air pressure therethrough with said second valve downstream of said irst valve; a governor device for each engine responsive to output shaft speed; each of said governor devices being connected to said second valve of its `associated fuel control means for varying the position thereof in response to output shaft speed; a iirst pressure responsive member for each engine connected to said first valve of its associated fuel control means to control the position of said rst valve in response to pressures acting thereon; said first pressure responsive member being connected on one side thereof to said air conduit means of its associated fuel control means upstream of said first valve and is thereby exposed to compressor discharge pressure of its associated engine; selecting valve means connected to said air conduit means of all engines in said `baril; of engines and operative to select the maximum compressor discharge pressure; said selecting valve means connected to each of said first pressure responsive members on the opposed side thereof so that said first valves are positioned in response to pressure comparisons; and each of said fuel control means including a second pressure responsive member connected to its associated air conduit means intermediate said iirst and second valves; said second pressure responsive member operative to vary fuel delivery to its associated engine in response to pressure sensed in said air conduit means.

'7. A load sharing system as claimed in claim 6 wherein said air conduit means of each engine includes a third air control valve in parallel flow relationship with said second air control valve; and speed limiting means responsive to compressor speed of its associated engine connected to said third air control valve to control the position thereof to limit compressor speed at a maximum safe value.

No references cited. 

1. A LOAD SHARING SYSTEM FOR A BANK OF TWO OR MORE TURBO-SHAFT ENGINES DRIVING A COMMON LOAD COMPRISING: A GOVERNOR DEVICE FOR EACH ENGINE; EACH OF SAID GOVERNOR DEVICES BEING RESPONSIVE TO THE SHAFT SPEED OF ITS ASSOCIATED ENGINE AND OPERATIVE TO CONTROL FUEL DELIVERY IN RESPONSE THERETO; ADJUSTING MEANS FOR EACH ENGINE; EACH OF SAID ADJUSTING MEANS CONNECTED TO ITS ASSOCIATED GOVERNOR DEVICE AND OPERATIVE TO ADJUST THE GOVERNOR CONTROLLED RELATIONSHIP BETWEEN SHAFT SPEED AND FUEL FLOW; POWER SENSING MEANS FOR EACH ENGINE; EACH OF SAID POWER SENSING MEANS OPERATIVE TO PRODUCE AN INDIVIDUAL POWER SIGNAL REPRESENTATIVE OF THE POWER DEVELOPED BY ITS ASSOCIATED ENGINE; SELECTING MEANS CONNECTED TO EACH OF SAID POWER SENSING MEANS FOR SELECTING THE MAXIMUM INDIVIDUAL POWER SIGNAL; COMPARISON MEANS FOR EACH ENGINE; EACH OF SAID COMPARISON MEANS BEING CONNECTED TO SAID POWER SENSING MEANS OF ITS ASSOCIATED ENGINE AND TO SAID SELECTING MEANS TO COMPARE ITS ASSOCIATED ENGINE INDIVIDUAL POWER SIGNAL WITH SAID MAXIMUM INDIVIDUAL POWER SIGNAL; EACH OF SAID COMPARISON MEANS BEING CONNECTED TO SAID ADJUSTING MEANS OF ITS ASSOCIATED ENGINE TO ADJUST SAID GOVERNOR DEVICE IN RESPONSE TO DEVIATION BETWEEN SAID INDIVIDUAL POWER SIGNAL AND SAID MAXIMUM INDIVIDUAL POWER SIGNAL. 